To meet the demand for wireless data traffic having increased since deployment of 4th generation (4G) communication systems, efforts have been made to develop an improved 5th generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Generally, mobile communication systems include base stations which provide wireless access in a cell of a predetermined range. The base station may be referred to as a different name according to the standard of a system. Each of the base stations provides wireless access to terminals located within the cell. Therefore, when a terminal moves from a cell of one base station to a cell of another base station, handover is performed to ensure continuity of services.
A normal handover procedure is as follows. First, a terminal determines whether a handover event occurs or not based on an intensity of a downlink signal. When the handover event occurs, the terminal reports the occurrence of the handover event to a serving base station, and the serving base station determines whether to perform the handover and then requests a target base station to perform the handover. Accordingly, the target base station determines whether to accept the handover and notifies the result of determining. The serving base station provides information necessary for performing the handover to the terminal, and thereafter, the terminal proceeds with a procedure for connecting to the target base station.
The handover normally occurs when the signal intensity of the target base station is greater than the signal intensity of the serving base station. This means that the terminal approaches the boundary of a serving cell. In this case, a channel gain between the terminal and the serving base station is low. In this case, when the signal intensity of the serving base station is greater than or equal to a predetermined level, the handover may be performed, but, when the signal intensity of the serving base station is low, a Radio Link Failure (RLF) may occur. As a result, it is highly likely that the handover occurring when the signal intensity of the target base station is higher than the signal intensity of the serving base station coincides with the ELF occurring when the channel gain between the terminal and the serving base station is low. In particular, in the case of a heterogenerous network where a small cell and a macro cell coexist, handover areas increase due to a small cell coverage of the small cell and high cell density, and furthermore, an interference between cells increases. Accordingly, areas satisfying the handover condition and the RLF condition also increase.
In the case of a 3rd Generation Partnership Project Long Term Evolution-Advanced (3GPP LET-A) system which is currently popularized, the handover and the RLF are declared when a predefined condition is maintained during a predetermined time. When the condition of the handover and the RLF is satisfied but is not maintained during the predetermined time, a pre-generated condition is invalidated. Herein, a procedure of declaring the handover and a procedure of declaring the RLF are independently defined and independently managed.
As described above, when the channel gain with the serving base station is low, the handover or the RLF may be declared. Since the procedure of declaring the handover and the procedure of declaring the RLF are independent from each other, when the terminal declares the RLF in the middle of proceeding with the handover procedure, the terminal may perform a connection re-establishment procedure regardless of the remaining handover procedure. Similarly, when the terminal declares the handover in the middle of proceeding with the RLF procedure, the terminal may perform the handover procedure. However, the satisfaction of the RLF condition means that the channel quality with the serving base station is poor, and thus the handover procedure including signaling between the terminal and the serving base station may not be smoothly performed.